Optical instrument and optical element providing expanded exit pupil

ABSTRACT

The invention provides an optical instrument including means ( 1, 2 ) to produce an optical image to be viewed by an observer including a diffractive element ( 7 ) located at an intermediate focal or image plane ( 4 ) of the optical instrument and comprising a pattern of a plurality of areas effective to cause diffraction interference in light passing through the optical instrument and thereby produce an expanded exit pupil ( 6 ) comprising a combination of a multiplicity of exit pupils displaced relative to one another transverse to an optical axis ( 3 ) of said instrument.

This invention relates to optical instruments and in particular tooptical instruments having an exit pupil at which an image of an objectmay be viewed by an observer.

In conventional optical instruments the size of the exit pupil isdetermined by a function of the numerical aperture of the instrument andthe overall magnification of the instrument and hence the size of theexit pupil is of fixed and relatively small dimension. Consequently itis necessary for an observer to accurately align the entrance pupil ofhis eye with the exit pupil of the optical instrument in order properlyto view an image or otherwise receive light from the instrument.

European patent application No. 94905777.2 discloses optical apparatusprovided with an enlarged exit pupil whereby an observer is able toenjoy the freedom to receive images or light from the optical apparatusby placing his eye pupil anywhere within an enlarged exit pupil. Theenlarged exit pupil is obtained by the provision of a diffractiveelement located at an intermediate focal or image plane of the opticalinstrument. The diffractive element comprises a substrate having aplurality of parallel first grooves in a surface of the substrate, edgesof the first grooves being coincident and forming lines of a firstdiffractive grating and a plurality of parallel second grooves in thesurface of the substrate, the second grooves extending perpendicular tothe first grooves and edges of the second grooves being coincident andforming lines of a second diffractive grating. While a construction ofdiffractive element as described in European patent application No.94905777.2 produces an enlarged exit pupil it has been found that thelight energy distribution over the extent of the enlarged exit pupil isnon-uniform. The light energy has a maximum energy level in a centralregion of the enlarged exit pupil and has an energy level that decreasesat locations spaced from the central region towards the periphery of theenlarged exit pupil. Furthermore if the diffraction element is designedto produce a more uniform energy level distribution over the extent of arequired enlarged exit pupil, there is significant light energy beyondthe extent of the required enlarged exit pupil and this energy is unusedand wasted.

According to one aspect of the invention an optical instrument includingmeans to produce an optical image to be viewed by an observer includes adiffractive element located at an intermediate focal or image plane ofthe optical instrument and comprising a pattern of a plurality of areaseffective to produce an expanded exit pupil comprising a combination ofa multiplicity of exit pupils displaced relative to one anothertransverse to an optical axis of said instrument.

According to a second aspect of the invention a diffractive element foruse in an optical instrument comprises a pattern of a plurality of areaseffective to produce diffractive interference of light passing throughor reflected by said element and thereby to produce an expanded exitpupil comprising a combination of a multiplicity of exit pupilsdisplaced relative to one another transverse to an optical axis of saidinstrument.

An embodiment of the invention will now be described by way of examplewith reference to the drawings in which:

FIG. 1 shows optical elements of a microscope incorporating adiffractive element,

FIG. 2 is a plan view of a part of a surface of a diffractive opticalelement,

FIG. 3 illustrates a profile of the diffractive element on the line 3—3of FIG. 2,

FIG. 4 shows optical elements of a projection microscope utilising atransmissive diffractive element,

FIG. 5 shows an alternative form of projection microscope using areflective diffractive element,

FIG. 6 shows an alternative construction of the projection microscopeillustrated in FIG. 5,

FIG. 7 shows the optical elements of a projection microscope utilising acombined Fresnel lens system and diffractive array,

FIG. 8 is a plan view of a part of a surface of an alternativediffractive optical element, and

FIG. 9 illustrates a profile of the diffractive element on the line 3—3of FIG. 8.

Referring first to FIG. 1, a microscope includes an objective lens 1 andan eyepiece 2 aligned on an optical axis 3. The objective lens producesan intermediate image in a focal or image plane 4 of an object in anobject plane 5. When an eye of an observer is aligned with an exit pupillocated at 6 a magnified image of the intermediate image and hence of mybe observed. A transmissive diffractive element 7 is located at theintermediate focal or image plane 4 of the microscope. In the absence ofthe diffractive element 7 an exit pupil of relatively small extent wouldbe produced at the location 6. However, the diffractive element iseffective to produce a multiplicity of exit pupils at the location 6displaced transversely of the axis 3 relative to one another. Incombination, the multiplicity of relatively displaced exit pupils forman expanded exit pupil that is of greater transverse extent than theexit pupil that would be formed in the absence of the diffractiveelement 7.

If desired the aperture of the object lens may be defined by an aperturestop 8. The aperture stop 8 may be circular and the exit pupil will thenalso be circular. However the aperture may be of a shape which is notcircular and for example may be rectangular, square or hexagonal. Themultiplicity of exit pupils that in combination form the expanded exitpupil are each produced with a shape corresponding to the aperture stop8. The transverse displacement of the exit pupils relative to each otherand the light energy in each of the multiplicity of exit pupilsdetermines the light energy distribution across the extent of theexpanded exit pupil. It is desired that the expanded exit pupil appearsto the eye of an observer as a single continuous expanded exit pupil.Furthermore it is desired that the exit pupils are so located transverseto the axis 3 of the microscope and that the light energy in each of themultiplicity of exit pupils is such as to produce a required lightenergy distribution across the extent of the expanded exit pupil.Usually it is desired that the light energy distribution across arequired extent of the expanded exit pupil is substantially uniform andthat at a peripheral edge of the required extent of the expanded exitpupil there is a relatively sharp decline in light energy so that thereis an insignificant level of light energy beyond the required extent ofthe expanded exit pupil. However if desired the light energydistribution may rise to a maximum in an area located centrally of theexpanded exit pupil whereby a viewer tends to be drawn toward an eyelocation aligned with the centre of the expanded exit pupil.

Referring now to FIG. 2, the diffractive element 6 includes a substrate9 having a surface 10, the surface 10 extending transversely to the axis3 of the microscope. A pattern 11 of a plurality of areas 12 is formedon the surface 10. Light passing through the diffractive element issubject to diffractive interference due to the presence of the areas 12and as a result, instead of the relatively small exit pupil that wouldbe formed in the absence of the diffractive element, a multiplicity ofrelatively transversely displaced exit pupils are formed that incombination form an expanded exit pupil. The pattern 11 of areas 12 mayextend across the whole of the surface 10 of the substrate or thepattern may extend over a part of the surface and be replicated over theremainder of the surface 10.

For purposes of illustration only, the areas are shown in FIG. 2 as ofrectangular shape and of different dimensions. However it is to beunderstood that the areas 12 may be of the same or similar dimensionsand shape or the areas may be of different predetermined shapes anddimensions located at predetermined locations in the pattern 11 providedthat the light passing through the diffraction element is diffracted insuch a manner as to result in the formation of a multiplicity of exitpupils that in combination form an expanded exit pupil. A furtherrequirement of the diffraction element is that the relative displacementof the exit pupils and the light energy level in each exit pupil formingthe expanded exit pupil is such as to result in a required light energydistribution across a required extent of the expanded exit pupil. Theareas are illustrated in FIG. 2 as being rectangular and defined bylinear sides extending in two mutually perpendicular directions. Howeverit is to be understood that the areas may be of different shape definedby sides which are non-linear, i.e. the sides may be arcuate, and thesides may extend in more than two directions to provide the requiredextent of expanded exit pupil having the required light energydistribution.

The areas 12 may be formed to be two-dimensional and extending on orimmediately adjacent the surface 10. The two-dimensional areas may beformed by deposition of an ink pattern or by exposure and subsequentdevelopment of an actinic photoresist. Alternatively the areas may beformed to be three-dimensional such that the areas 12 are projectionsextending to a predetermined height or heights from the remainder of thesurface 10, as shown in FIG. 3, or such that the areas are depressionslying at a predetermined depth or depths below the remainder of thesurface 10.

The pattern of areas 12 may be formed by various methods. For examplethe pattern of areas may be formed by holographic exposure of a laserwavefront interference pattern into an actinic photoresist deposited onthe surface 10 of the substrate 9. Another example of a method offorming the areas 12 is by direct writing of a Fourier transformpattern, using an electron beam, into actinic photoresist. Afterexposure the photoresist is developed to produce the required pattern ofareas 12.

While examples of methods of forming the areas 12 are disclosedhereinbefore, it is to be understood that these are provided by way ofexample and are not to be taken as limiting the invention to formationof the areas by these specific-methods.

It will be appreciated that when multichromatic light comprising lightof a plurality of different wavelengths is acted on by a diffractionelement, the diffraction of the light is dependent upon the wavelengthof the light. However it is often desired to operate a microscope andother optical instruments using multichromatic light. The formation of amultiplicity of relatively displaced exit pupils decreases observedcolour fringing effects and enhances the image observed by the viewer.Furthermore overlapping of the multiplicity of exit pupils tends tocancel colour fringing effects and thereby reduce the observed colourfringing.

Other forms of optical instrument utilising a diffractive element toproduce an expanded exit pupil comprising an array of exit pupils willnow be described with reference to FIGS. 4 to 7.

FIG. 4 shows a projection microscope including an objective lens 20, aprojection eyepiece 21 and field lenses 22, 23. In this construction ofprojection microscope the projection eyepiece images the aperture of theobject lens 20 or, if provided, of an aperture stop 24 to form anintermediate exit pupil at an intermediate plane 25. The field lenses22, 23 relay an image of the intermediate exit pupil at plane 25 to afinal exit pupil at location 26 for an observer. The objective lens 20and the projection eyepiece 21 form an image in a plane 27 intermediatethe field lenses 22, 23 of an object in an object plane 28. Atransmissive diffractive element 29 is located in the plane 27 toproduce a multiplicity of images of the intermediate exit pupil in plane25 such as to form an expanded final exit pupil at the location 26.

FIG. 5 shows a further embodiment of a projection microscope in which,instead of forming an image at a transmissive diffractive element as inthe microscope shown in FIG. 1 and the projection microscope shown inFIG. 4, the image is formed at a reflective diffractive element 30. Asingle field lens 31 and a reflective element 32 is provided to form animage for viewing by an eye 33 of an observer via a mirror 34. Adiffractive element 35 is provided adjacent the surface of thereflective element 32. The diffractive element 35 may be a separateelement as illustrated in FIG. 5 or may be integral with the reflectiveelement 32 and be formed on the reflective surface of the reflectiveelement 32. Instead of a field lens 31 and planar mirror 32, a concavepart-spherical reflective element 40 may be provided as shown in FIG. 6.A diffractive element 41 may be integral with the concave reflectiveelement and be formed on the part-spherical concave surface of thereflective element so that the diffractive element has the form ofcurvature of the surface of the reflective element.

Thus it will be understood that the invention provides an opticalinstrument including an optical diffractive element located at anintermediate image plane of the optical instrument which produces byreflective diffractive means or refractive diffractive means togetherwith an associated field lens or mirror system, a multiplicity of exitpupils forming in combination an expanded exit pupil at the viewingposition for an eye of an observer.

If desired, instead of using a refractive field lens, as shown in FIG.4, a Fresnel lens may be provided and the pattern of areas forming thediffractive element may be formed on a surface of the Fresnel lens. Thusas shown in FIG. 7, a single optical element 52 may perform thefunctions of the field lens system and of the diffractive element togenerate a multiplicity of relatively displaced exit pupils to form anexpanded exit pupil at the location 26.

It is envisaged that usually the diffractive element would remainstationary relative to the other optical elements of the opticalinstrument. However in some instances it may be desirable to move thediffractive element relative to the other optical elements of theoptical instrument for example by rotation of the diffractive elementabout an axis perpendicular to the plane of the element, or in the caseof a concave part-spherical element, about a central axis of theelement. Such rotation may be effected by providing an electric motorand a drive transmission from the motor to the diffractive element.

Hereinbefore the invention has been described in relation to microscopeshowever it is to be understood that the invention is not limited tomicroscopes and may be utilised to produce expanded exit pupils forother forms of optical instrument or apparatus.

As mentioned hereinbefore, the diffractive element may be a separateelement or may be formed integrally with another optical element such asa lens or mirror of the optical apparatus. When the diffractive elementis formed integrally with another optical element such as a lens ormirror, the other optical element acts as a substrate of the diffractiveelement and the pattern of areas is formed on a surface of the otheroptical element.

It is to be understood that where reference is made in the specificationto a lens, mirror or other optical element such element may comprise asingle optical element or a compound optical element consisting of acombination of elements.

1. A diffractive element for use in an optical instrument, comprising aplurality of replications of a pattern of separated diffractionstructures each effective to produce diffractive interference ofpolychromatic light passing through or reflected by the diffractiveelement, the plurality of diffraction structures including diffractionstructures of different sizes and shape, and the plurality ofreplications are configured to produce a plurality of exit pupils whichare displaced relative to one another transverse to an optical axis ofthe optical instrument such as to be viewable as a single, continuousexpanded exit pupil and which have a light energy such that a lightenergy distribution across an extent of the expanded exit pupil issubstantially uniform.
 2. A diffractive element as claimed in claim 1,wherein the diffraction structures of the plurality of diffractionstructures are two-dimensional.
 3. A diffractive element as claimed inclaim 1, wherein the diffraction structures of the plurality ofdiffraction structures are three-dimensional.
 4. A diffractive elementas claimed in claim 3, wherein the diffraction structures of theplurality of diffraction structures comprise projections.
 5. Adiffractive element as claimed in claim 4, wherein the projections areof one height.
 6. A diffractive element as claimed in claim 4, whereinthe projections extend to more than one height.
 7. A diffractive elementas claimed in claim 3, wherein the diffraction structures of theplurality of diffraction structures comprise depressions.
 8. Adiffractive element as claimed in claim 7, wherein the depressions areof one depth.
 9. A diffractive element as claimed in claim 7, whereinthe depressions lie at more than one depth.
 10. A diffractive element asclaimed in claim 1, wherein the plurality of replications are such thatthe plurality of exit pupils are overlapping.
 11. An optical instrumentfor producing an optical image to be viewed by an observer, the opticalinstrument including the diffractive element of claim 1 located at anintermediate focal or image plane thereof.
 12. An optical instrument asclaimed in claim 11, including an optical element, and wherein thediffractive element is formed on or is integral with a surface of theoptical element.